1. Field of the Invention
The present invention relates to a solid-state imaging device of CCD (charge coupled device) type in which a charge transfer path (transfer register) is branched into parallel two paths at an output end, a driving method therefor and an imaging apparatus, and more particularly to a two-branch output type solid-state imaging device suitable for improving the quality of a captured image, a driving method therefor and an imaging apparatus.
2. Description of Related Art
Recent solid-state imaging devices of CCD type have shown an increase in the number of pixels along with the progress in the semiconductor micro-fabrication technology, and have reached a level of containing several million pixels. It is therefore required to increase the transfer drive frequency for a horizontal transfer path for reading out signals, thus leading to a drawback that a voltage signal output from an output amplifier is distorted in waveform.
Therefore developed is a technology of branching an output end of the horizontal transfer path in parallel manner, performing the transfer in the horizontal transfer path with a high-frequency signal, and, in a distributing part (branching part), distributing signal charges, transferred in succession through the horizontal transfer path, to successively distribute to the branched transfer paths, thereby reducing a drive frequency of the branched transfer paths lower than the drive frequency of the horizontal transfer path (for example see JP-A-5-308575, Japanese Patent No. 2949861 and Japanese Patent No. 2624138).
A technology in JP-A-5-308575 describes a construction of branching the output end of the horizontal transfer path into three, and distributing and outputting an R (red-color) signal charge, a G (green-color) signal charge and a B (blue-color) signal charge respectively the branched transfer paths, whereby the drive frequency of the branched transfer paths is made ⅓ of the drive frequency of the horizontal transfer path.
However, in a solid-state imaging device containing several million pixels or higher and capable of capturing a high-definition image for example of high-vision quality, the horizontal transfer path has to be driven with a considerably high frequency, so that it is difficult, in the distributing part, to clearly distribute the signal charges into three ways while maintaining a high transfer efficiency (namely without a residual charge in charge transfer).
Technologies in Japanese Patent No. 2949861 and Japanese Patent No. 2624138 propose a solid-state imaging device of two-branch output type. Such two-branch output type has advantages, in comparison with the three-branched output type, of an easier distribution of the signal charges at the distributing part and a drive frequency for the horizontal transfer path only as high as twice of the drive frequency for the branched transfer paths.
However, Japanese Patent No. 2949861 and Japanese Patent No. 2624138 do not describe transfer of signal charges of a color image. In such two-branch output type solid-state imaging device, the branching part distributes signal charges for example of red and blue colors to a branched path and a signal charge of green color to the other branched path. Therefore, in the case that a residual charge is generated in the branching part, a branching residual charge, generated in a previous stage and corresponding for example to red color, is mixed in a signal charge, to be branched in a next stage and corresponding to green color, thereby deteriorating the quality of the captured image.
A two-branch output type solid-state imaging device has an advantage of elevating a data rate in a horizontal transfer path as a drive frequency after branching can be reduced to ½, but is associated with a drawback that the image quality of a captured image is deteriorated when a residual charge is generated in branching the signal charge.